1. Field of the Invention
The subject invention relates to electric motors and, electric motor controls and control methods and systems.
2. Disclosure Statement
This disclosure statement is made pursuant to the duty of disclosure imposed by law and formulated in 37 CFR 1.56(a). No representation is hereby made that information thus disclosed in fact constitutes prior art, inasmuch as 37 CFR 1.56(a) relies on a materiality concept which depends on uncertain and inevitably subjective elements of substantial likelihood and reasonableness, and inasmuch as a growing attitude appears to require citation of material which might lead to a discovery of pertinent material though not necessarily being of itself pertinent. Also the following comments contain conclusions and observations which have only been drawn or become apparent after conception of the subject invention or which contrast the subject invention or its merits against the background of developments subsequent in time or priority.
In recent years, mechanical actuators have reached a high state of perfection. By way of example, reference may in this respect be had to U.S. Pat. No. 4,174,575, by Kyohiro Nakata, issued Nov. 20, 1979, for Measuring Instrument, and now assigned to the assignee of the subject patent application or patent. One of the measuring instruments disclosed in that patent is a differential micrometer having an adjustment precision of one-half micrometer or 0.0005 millimeters. While that micrometer is, of course, very useful as a measuring instrument, it also serves as a high-precision actuator in such delicate applications as optical component adjustment in holographic or other laser beam utilization systems. A disadvantage of using differential micrometers as component actuators is, however, that they are manually operated.
Attempts have thus been made to provide electrically energized actuators which could take the place of the manually operated micrometer type. Accordingly, a small electric motor with reduction gearhead has been accommodated in a tubular housing, to be axially movable therein. The output shaft of the gearhead was attached to a threaded spindle for translating rotary motion into translatory motion.
This, in turn, called for some limit sensing or handling system. In the past as well as at present, limit switches were and are frequently employed to control the travel of translatorily moving or other components. Such switches, however, have the disadvantage of requiring substantial space, which is a particular design impediment in micrometer and micro-actuator devices.
Also, limit switches require special wiring and connections to a control apparatus and are vulnerable to wear and tear and prone to failure before the normal life span of the remainder of the equipment.
The latter limitations of limit switches are also felt as a drawback in applications other than those so far specifically mentioned. For instance, limit switches often present a danger when translatory or rotary drives are used in explosive or humid environments, where the danger of setting off explosions through electric sparks or electric shocks through insulation failure is particularly prevalent.
Some of these disadvantages may be avoided by the use of rotary encoders, which are also capable of providing overload and stall sensing functions. By way of example, a lengthening of an encoder pulse period as the motor slows down at a limit, stall or severe overload condition may be sensed for control purposes. In particular, if the pulse period is longer than a fixed time threshold, one term of two-term limit sensing function is activated. The second term is activated if the motor driving voltage is higher than a preset positive threshold or lower than a preset negative threshold. The motor driving voltage is typically created by a velocity servo which forces the voltage upward to overcome increasing loads when traveling forward. When the motor driving voltage is higher than a positive threshold and the encoder period is longer than a certain time, a forward limit is indicated. Conversely, when the motor driving voltage is lower than a negative threshold and the encoder period is longer than a certain time, a reverse limit is indicated.
In a prior-art apparatus embodying the latter functions, the active element governing the encoder period was a retriggerable multivibrator that was reset to a non-limit condition each time an encoder transition was input. The retriggerable multivibrator had a time out window of approximately 2.5 milliseconds. The time-window was determined by fixed resistors and capacitors. In order to avoid indication of a limit a short time after just starting the motor, a capacitor was provided for delaying the second limit-sensing term, with the capacitor causing the circuit to wait until the motor reached commanded speed and an encoder transition had occurred.
The major deficiency of that prior-art limit sensing circuit was the fixed time threshold on the retriggerable multivibrator, which meant that the motor had to slow down to the same velocity no matter how fast it was originally going before the limit was sensed.
Another deficiency was that the prior limit sensing circuit required a rotary encoder; a relatively expensive component requiring extra space at the motor or actuator and leads going to the sensing circuit.